This invention relates to frequency discriminators and their applications and particularly to a frequency discriminator having a variable bandwidth over a wide range in which the center frequency is simultaneously tunable over an extremely wide bandwidth. Although the invention is particularly useful in microwave frequency applications it is not limited to such uses. Also, while the preferred embodiments disclosed herein typically employ ferrimagnetic resonators, such as YIG (yittrium-iron-garnet), other elements having similar characteristics may be used subject to the performance characteristics of such other elements.
Center frequency tuning of prior art resonant circuit discriminators consists of basically two different types: mechanically or electronically tuned center frequency. Both types use either the amplitude or phase versus frequency characteristics of a single or dual resonant circuit to indicate the frequency of the input signal relative to the discriminator center frequency. In the amplitude comparison mode, a dual mode resonant circuit is preferred with dual detectors. In the phase comparison mode, a single resonant circuit is often sufficient with dual detectors used to convert phase to amplitude. The output of such prior art discriminators is very sensitive to the amplitude of the input signal, and they require critical and expensive means to lessen such amplitude dependence.
Because of the amplitude comparison between dual detectors required in prior art approaches, improving the discriminator's capability to detect low level signals is only possible by introducing signal gain at the carrier frequency. The critical problems of tracking detectors over a wide dynamic range and the need for tracked high gain video amplifiers makes application after detection impractical.
The bandwidth of the prior art discriminators is generally fixed by the loaded Q of the resonant circuit and/or the circuit coupling factors in the case of dual mode resonators. In many applications, compromises must be made between the desired discriminator bandwidth, linearity and resolution. For example, a wide bandwidth is often desirable to ensure that the input frequency falls within the discriminator range; while a narrow bandwidth is desirable to provide the best possible frequency resolution. It is also difficult to achieve wide bandwidths and good linearity simultaneously because of the phase and amplitude non-linearity of the resonant circuit. Another characteristic of the prior art discriminators is the difficulties in achieving a wide tuning range of the center frequency. The limitations of mechanically tuned circuits are due to the fact that physical dimensions have to be changed and often multiple resonant circuits must be tracked to maintain constant discriminator bandwidth. In the microwave frequency region, this limitation has been overcome to some extent by substituting ferrimagnetic cavities that can be electronically tuned over a wide bandwidth. Thus, yittrium-iron-garnet (YIG) discriminators have been built in the manner of Nathanson (U.S. Pat. No. 3,274,519) using amplitude comparison or Goodman et al (U.S. Pat. No. 3,364,430), Hoover et al (U.S. Pat. No. 3,562,651) and Pircher (U.S. Pat. No. 3,622,896) using phase characteristics of YIG resonator. In the dual mode amplitude comparison approaches, center frequency tuning range is limited by ability to divide input amplitude equally between two cavities and the ability to track cavities to maintain a constant bandwidth. In the phase reference approach, the center frequency range is limited by the bandwidth of broadband phase shift networks necessary to establish proper phase reference and the ability to equally divide input power. Current practice limits center frequency tuning range to about a single octave.
Both YIG discriminator approaches have narrow, fixed bandwidth characteristics whose output voltage versus input frequency slope is sensitive to input signal amplitude and center frequency changes, and whose linearities are difficult to maintain because of impedance mismatches and spurious resonant nodes in the ferrimagnetic resonator. Both discriminator types require dual detector outputs and expensive and bulky ancilliary microwave components; both require extensive critical alignment to cover wide tuning range with near constant discriminator slope.
In the specific prior art approaches applied to automatic frequency control, a single, fixed center frequency mechanically tuned cavity has been used to stabilize the frequency of high frequency generators. In one method of approach for this application, a cyclical mechanical modulation of the center frequency of the cavity has been used to sense the position of the generator frequency relative to the cavity center frequency and provide a correction signal to control the generator. The purpose of this approach was to use the superior mechanical stability of the cavity to stabilize the frequency of the generator. The cavity was tuned sinusoidally by mechanical means and the rate of tuning was limited to slow variations inherent in mechanical variations of the cavity. The application of the present invention in automatic frequency control is to vary the center frequency of the generators by tracking them to the variable center frequency of the discriminator. Bandwidth adjustments on the discriminator can be made to facilitate the initial capture of the generator and then to maximize the frequency resolution. The sample rate of the discriminator is electronically controlled and can be optimized to match the desired tuning rate of the generators.